[Haskell-cafe] deriving Data.HashTable - stack overflow

[Erik Hesselink]

There is a ticket with discussion and a patch here [0].

Erik

[0] http://ghc.haskell.org/trac/ghc/ticket/7633

On Thu, Aug 8, 2013 at 8:11 PM, David Thomas <davidleothomas at gmail.com> wrote:
> I do wish there was a compiler-checked way of specifying a minimum complete
> definition.
>
>
> On Thu, Aug 8, 2013 at 11:02 AM, Joey Adams <joeyadams3.14159 at gmail.com>
> wrote:
>>
>> On Thu, Aug 8, 2013 at 12:22 PM, Lyle Kopnicky <lists at qseep.net> wrote:
>>>
>>> ...
>>>
>>>
>>> So I went to the Data.Hashable page and looked up examples on how to
>>> derive a Hashable instance for my datatype:
>>>
>>> http://hackage.haskell.org/packages/archive/hashable/latest/doc/html/Data-Hashable.html
>>>
>>> The problem occurs even when using the sample code on the page:
>>>
>>>
>>> {-# LANGUAGE DeriveGeneric #-}
>>>
>>>  import GHC.Generics (Generic)
>>>  import Data.Hashable
>>>
>>>  data Colour = Red | Green | Blue
>>>                deriving Generic
>>>
>>>  instance Hashable Colour
>>>
>>> If I then type `hash Red` I get a stack overflow.
>>>
>>> I am using the Haskell Platform, so I have hashable-1.1.2.5, but I notice
>>> the docs are for hashable-1.2.0.10. If I install 1.2.0.10 though, other code
>>> in my project breaks - seems like one part doesn't recognize the instances
>>> from another part. So I'll stick with the platform version.
>>>
>>> ...
>>
>>
>> Generic support was added in hashable-1.2.  Before then, the default
>> implementations for `hash` and `hashWithSalt` were written in terms of each
>> other:
>>
>>     hash = hashWithSalt defaultSalt
>>     hashWithSalt salt x = salt `combine` hash x
>>
>> Because you did not give an implementation for either of these, both
>> default implementations were used, leading to a loop.
>>
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>
>
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